Re: [Haskell-cafe] ANN: monad-bool 0.1

If we step back, I think the lesson here is that Haskell libraries exist, but the concepts are far enough from what you expect to exist given background knowledge from another programming language. So what is actually needed is not monad-bool, but the equivalent documentation that makes a programmer which is not an expert in Haskell connect the dots like you did today. Alexander On Wed, Jan 23, 2013 at 8:10 AM, John Wiegley wrote:

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John Wiegley writes:

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Never mind, when/unless + left work just fine for this.

You know, it's been a humorous day.

First ekmett showed that I can't make a sane Monad instance for AndM or OrM.

Then I discovered I can't make a reasonable Monoid (no mempty, given only 'toBool'), so I dropped down to a Semigroup. Further, my combinators for EitherT can be implemented using just 'when' and 'left' to provide the short-circuiting.

Already I had very little code left, until he showed me the Applicative instance for Either, plus a little trick:

>>> Right 1 *> Right 2 *> Left 2 *> Right 5 Left 2 -- same functionality as my And semigroup

>>> let Left x |> y = y; x |> _ = x in Left 1 |> Right 2 |> Right 3 Right 2 -- same functionality as my Or semigroup

And poof, all my code just disappeared...

-- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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